The
Sceptics Say: ”Global Warming has been going on
for 20,000 years.”

This one
varies in the length of time quoted, but the essential fallacy is the
same. You can cherry pick any particular year you wish to get a trend
in the direction you wish. But of course this only works if you
ignore all context or considerations of relevance.

Answer:

It is quite true
that 20K years ago the temperature was some 8 to 100C
colder than it is today, but it is highly arbitrary and dubious to
simply draw a line from that point to today and say

"Look!
20,000 years of Global Warming!"

If you have a look
at the graph of temperature, starting at a point when we were
finishing the climb out of deep glaciation, you can clearly see that
rapid warming ceased around 10,000 years ago (rapid relative to
natural fluctuations, but not compared to warming today). After a
final little lift at 8000 years the temperature trended generally
downward for the entire period of the Holocene. So
the post industrial revolution warming is really the reversal of a
many thousand year trend.

A closer view of
today's
trend
with the context of the last 1000
and 2000
years makes it even clearer that today's trend is striking and
opposite to what one might expect without an anthropogenic
disturbance.